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Building a coilgun usually means working with SCRs as power switches. In that application, one needs to consider not the average on-state current rating I_T, bur the peak non-repetitive surge current I_TSM. This last parameter, states "1/2 cycle, sine wave 60 Hz, T_J = 100 C". Does that means that the rated surge current can pass through the device for half a 60 Hz cycle, that is for 1/120 of a second? Thanks in advance
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im trying to remember the SCR stuff (these vary things your looking at) form the late 90's I learned. back before we had all these nice IGBTs and Later generation Mosfets...
the type of SCR your describing is called a phase control SCR, meaning like a dimmer switch, they chop a sine or similar repeating wave...
so when the junction is at 100C, they rate yours for a pulse 1/120 of a second, but they typically mean through a resistive load. (for you this is important, a coils Resistance will be low and shed V across the SCR junction, which will decrease there claimed number in your application.) but you wont be operating for 20 years at 90C, so that favors your case a bit.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
There is a slight correction factor because Itsm is for a sinusoidal half-cycle, but the way you worded things implies that you want to use a rectangular pulse. So you should really multiply Itsm by pi/4 or whatever.
In practice for your coilgun, the pulse won't be rectangular and the starting junction temperature will be less than 100c, so the uncorrected Itsm value should be conservative.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
There is a slight correction factor because Itsm is for a sinusoidal half-cycle, but the way you worded things implies that you want to use a rectangular pulse. So you should really multiply Itsm by pi/4 or whatever.
In practice for your coilgun, the pulse won't be rectangular and the starting junction temperature will be less than 100c, so the uncorrected Itsm value should be conservative.
yep, I was going to make your first point. but had class... the SCRs are robust but phase correction SCRs are less ideal in this app. Since I presume he'll be using caps discharged through his coils the waveform could drastically change between projectile present, and projectile absent from the coil, right?
or is this one of those single flat coil disc shooters?
the real problem is that SCRs don't have an isolated gate, they take current to turn on, and they can kick back out of the gate and kill non-isolated gate drive electronics, problems not seen with IGBTs/Mosfets gates.
This will be for a single-stage coilgun. Porblem is, i don't have easy access to igbt/mosfets. Actually in order to save money i plan to parallel several SCRs. I have a bunch of 2N6508's. I plan to use a cap bank of photoflash electrolitycs, 2500 uF @ 500 volts.
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Understanding SCR parameters
